The present invention relates to flying spot scanners, which are used as raster input scanners, or as raster output scanners (ROS). The invention is particularly applicable to ROS's used for electronic image transmission and processing, such as laser printing and facsimile.
In a conventional ROS, a modulated optical beam is raster-scanned across a photoreceptor, with the light spot being rapidly scanned horizontally, thus forming a horizontal scan line, and the light spot being simultaneously scanned vertically at a slower speed. The photoreceptor may be an electrostatically charged drum, or some other form of photosensitive imaging device. Horizontal scanning may be provided by a rotating wheel having a number of flat mirror facets on its periphery. Vertical scanning may be provided by steering the optical beam, but it is usually provided by moving the photoreceptor itself in a vertical direction. As shown in FIG. 1, the continuous horizontal scanning has the effect of smearing out and blurring each exposed pixel horizontally, and thereby decreasing the resolution of the scanner's output image. This effect occurs when there is a transition from one exposure level to another, e.g. from exposure on to exposure off or from exposure off to exposure on. FIG. 1a shows a desired pattern as it would be laid along a scan line. FIG. 1b shows how this segment of a scan line would ideally be written using a string of round pixels, corresponding to the sequence in FIG. 1a: 2 pixel exposures followed by alternating exposures off and on. The first pixel exposure begins at time 14 and ends at time 16; the second pixel exposure begins at time 16 and ends at time 18, blending into one exposure interval. The third pixel exposure begins at time 20a and ends at time 20b, and the fourth pixel exposure begins at time 21a and ends at time 21b.
FIG. 1c shows how blur occurs in a conventional flying spot scanner due to movement of the spot during exposure intervals. The spot is centered at position 14 at the start of the first exposure interval, and centered at position 18 at the end of this exposure interval. Similarly, the spot is positioned at 20a and 21a at the start of the second and third exposure intervals, respectively; and at positions 20b, and 21b at the end of the second and third exposure intervals, respectively. FIG. 1d shows a typical exposure profile that would result from positioning spots at the locations indicated by FIG. 1a, and maintaining these positions during the exposure intervals. FIG. 1e shows a typical exposure density profile that results from a scanning spot that moves during the exposure intervals as shown in FIG. 1c. In the blurred image of FIG. 1e, the transition gradient between exposed and unexposed areas is reduced relative to the unblurred image of FIG. 1d, and the contrast between adjacent single exposed and unexposed pixels is reduced relative to the unblurred image of FIG. 1e.
In order to overcome the effects of exposure blur it is common practice in flying spot scanning systems to utilize an elliptical spot rather than a round spot as shown in FIG. 1. The small axis of the ellipse is oriented in the horizontal scan direction, so that the blurring effect in the horizontal creates a more or less round exposure. However, there are undesirable side effects to using elliptical scanning spots: the f-number of the spot producing beam must be decreased (increased input beam size) in the horizontal axis. This increased beam size requires an equal increase in the size of the scanning mirror facet, and the decreased f-number also causes the depth of focus to be decreased which makes mechanical alignment more difficult. Also, some scanning systems cause the spot to rotate as it is scanned, which thus precludes use of an elliptical spot for exposure blur compensation.
The present invention causes each individual pixel to be stabilized on the photoreceptor while the pixel is being written, so that there is no smearing of pixels due to beam motion. The present invention also enables the output beam of the scanner to move rapidly from pixel to pixel.
The problem of providing high quality output from raster output scanners has presented a major challenge to designers. Improved resolution would provide higher quality output than is presently available from flying spot scanners/raster output scanners. There is a long felt need within the flying spot scanner industry for a scanner which will output distinct, well-defined pixels.